Frequency filters are elements capable of allowing passage of a determined frequency range of an alternating type signal, for example an electromagnetic wave or an acoustic wave.
Frequency filters are particularly used in elements intended for high frequency applications. For example, they are found in multiplexers, diplexers, amplifiers, oscillators or mixers.
High frequencies are useful because they can carry a large amount of information. Furthermore, the increase in frequency can significantly improve the resolution of detection devices and can miniaturise systems. Thus, there are many high frequency applications; for example they are used in broadband radio communications and inter-satellite radio communications (frequency about 60 GHz), in anti-collision radars (frequency 70 GHz) and radiometry (frequency 180 GHz).
The document “Integrated millimetre-wave silicon micromachined filters”, written by the IRCOM, the CEA and the CNES, in October 2000 presents an embodiment of micro-machined filters for high frequency applications.
FIGS. 1 and 2 represent 2-pole filters or resonators made using the technique described in this document. These filters comprise a structure (substrate 100) that can be decomposed into several areas:                at least two areas 1a, 1b acting as dielectric resonators,        at least one wave propagation area 2 located between two resonators and acting as a wave guide,        two extreme areas 3 forming evanescent areas.        
The dielectric resonators 1a, 1b are made from dielectric materials with a high relative permittivity. This high permittivity confines electric fields in the resonator. Resonator sizes must be chosen to fix the required operating frequency of the filter, as is known to those skilled in the art.
The cavity-shaped propagation area 2 forms a wave guide that couples the two resonators 1a and 1b. The guide dimensions act on the coupling factor and on the frequency of the filter obtained.
Finally, the two extreme cavity-shaped areas form two evanescent areas that have the function of eliminating reflections of parasite waves. To be efficient, these evanescent areas must be longer than the waves circulating in the filter.
At the present time, frequency filters are made by successive deposition and etching steps. The disadvantage of this operating method is that it limits the possibilities of making filters and also restricts their performances.
The filter illustrated in FIG. 1 is composed of a structure comprising three cavities and arranged on a metallisation layer 6. In general, the filter structures are micro-machined in a high resistivity silicon substrate 100, because this is a material with low dielectric losses at millimetric wavelengths and a high permittivity, which makes it possible to make miniaturised filters.
Once the filter structure is terminated, it is covered with an electromagnetic shielding. This shielding avoids the dispersion of waves in the filter or accidental escape of the waves. The shielding consists of three metallisations: a metallisation 5 on the front face 7 of the structure, a metallisation 5 on the back face 8 of the structure, and a metallisation 6 on which the structure of the filter will be placed. This metallisation 6 is often a metallisation layer deposited on a host substrate. The different metallisations are connected together to close the shielding around the filter to make the contact between the front face 7 and the back face 8 of the filter structure by metallisation of four edges 9 or sides that surround the structure, and by making the contact between the back face 8 and the host substrate, on which the metallisation layer 6 is placed, through a fusible alloy 10 (fusible balls). The filter is transferred using fusible balls 10 onto a host substrate using the flip-chip or an equivalent method. The micro-machined filters installed in flip-chip have the advantage that they can then be integrated into more complex subassemblies.
FIG. 2 shows that the filter structure comprises at least two coupling windows 4 that couple resonators with the metallic tracks 6 of the host substrate.
There are many disadvantages related to micro-machined filters like those described in prior art, related to their manufacturing method and their performances.
As has already been seen, several areas of the silicon substrate 100 must be etched to make a frequency filter. The cavities that will form the wave guide and the evanescent areas that surround the resonators, and the four edges that surround the filter structure and enable contact of the shielding between the front face and the back face of the filter structure, have to be etched. The surface hollowed out from the remaining surface of the substrate makes the substrates fragile when the filters are being made. The number of structures made on each substrate wafer has to be reduced, to prevent substrate wafers from breaking.
The filters are held in place during manufacturing by support beams 12 in the substrate. These support beams are preferably placed at the corners of the filter (see FIG. 2) to minimise parasite effects related to breakage of the shielding at these areas. When these beams are cut out, the shielding on these beams must be cut to release the filters, and it reduces the performances of the filter.
Furthermore, etching is usually done by wet etching to make a micro-machined filter. Since the internal cavities of the filter, in other words the wave guide(s) and the two evanescent areas, do not necessarily have the same dimensions, the cavities cannot be made by plasma etching. Plasma etching is specific in that it has an etching rate that depends on the surface of the pattern; therefore, it is impossible to have the same etching depth for two different pattern sizes. Wet etching of silicon is an anisotropic etching that follows the <111> crystalline plane of silicon at an angle of 54.7° from the <100> crystalline plane. An alignment error of 10 causes a loss of dimension of 175 μm for a structure length of 1 cm. Thus, the dimensions of the patterns to be etched and the precision of the searched alignments make it impossible to reproduce the filters. Since the resonant frequency and the quality factor of a filter depend on the dimensions of its cavities and its resonators, the performances of filters vary from one filter to another.
Furthermore, as can be seen in FIG. 1, a layer of dielectric material 11 is deposited between the silicon 100 from which the filter structure is made and the metallisation layer 5, to isolate the substrate from the metallisation layer. Note that this layer of dielectric material 11 should ideally be present under the entire metallisation layer 5 to provide the best performances. However, for reasons of simplification of the technology, it is only provided at the connection with the host substrate. For hyperfrequency applications, it is desirable to use dielectric materials with low dielectric losses. For example, SiO2 deposited by PECVD will be chosen, which generally has lower dielectric losses than thermal SiO2. These dielectric materials must also be only slightly stressed to prevent a large deformation (sag) of the filter structure that would prevent the operation to assemble it with the host substrate. For example, assembly is not possible if the deformation of the structure is greater than the difference in height of the fusible balls. Furthermore, this dielectric material must be used as a mask during etching of the substrate. Dielectric materials that are interesting for hyperfrequency applications are not necessarily adapted to wet etching of silicon. Thus, there is a very small choice of dielectric materials.